Field of the Invention
The invention relates in general to photomasks for lithographic exposure and structuring methods with the aim of producing microscopic material structures such as photoresist structures and, on this basis, of producing microscopic material structures such as magnetic memory element configurations or the like. In particular, the invention relates in this case to phase masks and their utilization in these methods and the production of dimensionally critical material structures.
For the production of the memory level (XMR level) of magnetic memory components such as magnetic RAM data memories (MRAM), as described for example in S. Mengel: Technologieanalyse Magnetismus Band 2, XMR-Technologien [Technological analysis, magnetism volume 2, XMR technologies], published by VDI-Technologiezentrum Physikalische Technologien, 1997, it is a general object to produce the individual magnetic memory elements in increasing densities, in order to increase the storage capacity of the entire memory component. Hitherto, such structures have been produced either electron-optically by means of a direct writing method or by means of conventional optical lithography, corresponding to the conventional methods for producing microelectronic circuits.
In photolithographic processes, the structures are projected optically onto light-sensitive layers such as photoresist layers on a substrate in a conventional manner by photomasks. Because of the diffraction effects, the resolution power of such a projection system is limited and mask structures having dimensions below the inverse value of this resolution capacity, the dimensionally critical structures as they are known, become blurred or projected unsharply. In order to be able to produce magnetic memory elements with high density, it is previously necessary for a photoresist layer in the form of a matrix-like configuration of resist clusters or dots or of cutouts or depressions in a resist layer to be structured. Using conventional lithography, however, it is very difficult to reach below the resolution limit which is normally given, wherein half the distance between two resist dots is given by k1xcex/NA with (k1≈0.38, xcex the carrier wavelength of the illumination, NA the numerical aperture). At the least, such dense resist dots cannot be produced with a non-negligible process margin in the conventional optical way. Furthermore, the conventional optical projection is very sensitive to fluctuations in the mask dimensions which, for example, can be described by the xe2x80x9cmask error enhancement factorxe2x80x9d (MEF).
The above problems constitute limiting factors for the cost-effective and competitive fabrication of MRAM memory components with critical dimensions below 100 nm with conventional lithography and mask technique.
These difficulties may be overcome with phase masks, as they are known. In phase masks, the destructive interference effect of two closely adjacent and coherent light beams with phases shifted by 180xc2x0 is utilized.
The various types of phase masks are described, for example, in the book xe2x80x9cTechnologie hochintegrierter Schaltungenxe2x80x9d [The technology of highly integrated circuits] by Widmann, Mader, and Friedrich, 2nd edition, Springer-Verlag, pages 135ff. An extensive overview of phase mask technology is given in the publications xe2x80x9cImproving Resolution in Photolithography with a Phase-Shifting Maskxe2x80x9d by M. D. Levenson et al. in IEEE Trans. Electron. Devices 29 (1982), 1828ff. and xe2x80x9cWavefront Engineering for Photolithographyxe2x80x9d by M. D. Levenson in Physics Today, July 1993, p. 28ff.
In the case of MRAM memory components, it is additionally advantageous to produce the individual magnetic memory elements as elliptically shaped structures of high density, since they thus impart a preferred direction to the magnetic storage medium. This has been shown in the publications xe2x80x9cGiant magnetoresistance by melt-spun CUxe2x80x94CO alloysxe2x80x9d by J. Wecker et al. in Appl. Phys. Lett. 62 (1993), pp. 1985-1987, and xe2x80x9cGMR angle detector with an artificial antiferromagnetic subsystem (AAF)xe2x80x9d in J. Magn. Mat. 165 (1997) p. 524, using magneto-resistance elements.
Conventional optical lithography is also problematic under this last-named aspect. This is the case because using the conventional binary chromium photomasks, defined production of elliptical structures, that is to say both depressions (holes) in resist layers and resist dots, can generally not be carried out. In the case of dense structures, the necessary reserve on the mask structures would, under certain circumstances, be too great for practical mask production.
It is accordingly an object of the invention to provide a method for lithographic structuring of a material layer, in particular a photoresist layer, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which allows microscopic, in particular elliptical material structures to be produced. In particular, it is an object of the present invention to specify a suitable photomask and its use for implementing such a method. Furthermore, it is an object of the present invention to provide a method on this basis of producing a magnetic memory component such as an MRAM memory component with the aid of the structuring method.
With the foregoing and other objects in view there is provided, in accordance with the invention, a photomask for lithographic structuring methods, comprising:
a configuration of at least partially transparent strip-like areas arranged immediately adjacent and parallel to one another; and
wherein adjacent areas are formed such that rays of light passing therethrough have a phase difference of 180xc2x0 relative to one another.
With the above objects in view there is also provided, in accordance with the invention, a lithographic structuring method which comprises providing a material layer, in particular a photoresist layer, and exposing the material layer through the above-summarized photomask.
Finally, there is also provided a method of producing a matrix-like configuration of magnetic memory elements, which comprises:
producing a matrix-like arrangement of microscopic photoresist structures in accordance with the above method; and
subsequently producing the matrix-like configuration of magnetic memory elements with the aid of the arrangement of microscopic photoresist structures.
A significant idea of the invention consists in using at least one photomask, which is formed as a phase mask, in the exposures of a material layer to be structured, in particular a photoresist layer. Such a photomask has an arrangement of at least partially transparent strip-like areas, which are arranged immediately adjacent and parallel to one another, adjacent areas being formed in such a way that rays of light passing through them have a phase difference of 180xc2x0 from one another.
Using such a photomask, a first exposure can be carried out, narrow strip-like sections remaining unexposed on the surface of the material layer to be exposed, because of the destructive interference of the rays of light passing through adjacent areas, said sections being defined by the boundaries between the adjacent areas.
A second exposure can then be carried out, wherein, for example, a second corresponding photomask is used, the alignment of the strip-like arrangement of the photomasks being rotated through an angle, in particular 90xc2x0, between the exposures. At the crossing points of the strip-like arrangements, hole-like or cluster-like resist structures are formed, so that the result produced is a matrix-like arrangement of such resist structures.
The novel method has the advantage that hole-like or cluster-like material structures such as resist structures can be produced, their extent, at least in one direction, falling below critical dimensions. For example, the case may occur wherein structures are required which have to be subcritical only in one direction but in the other direction have an extent above the critical dimensions. With respect to the last-named direction, it is possible for example for a conventional, binary chromium photomask to be used, which has an arrangement of strips of chromium running in this direction, their width corresponding to the structure width desired in this direction. In this way, for example, elliptical structures can be produced wherein the minor axis of the ellipse is dimensionally critical but the major axis is not.
Likewise, it may be desired for the resist structures to be produced to be dimensionally critical in both directions. In this case, in practical terms, a single phase mask can be used and, after a first exposure step has been carried out, can be rotated through a predetermined angle, a second exposure step then being carried out. In this case, both exposure steps can be carried out identically, so that in the ideal case circular cluster-like or hole-like resist structures with dimensionally critical extents are produced. However, even for this case, it may be desired for elliptical structures to be produced which are dimensionally critical in both directions. This can then be achieved by choosing different exposure doses and/or different filling factors ("sgr" or NA) for the various exposures and/or deliberately causing a projection error such as coma or astigmatism during at least one exposure and/or by introducing an oblique course of the phase boundary on one of the phase masks used during the exposures.
In the case where phase masks are used, chromium-free phase masks are preferably used. With these, in principle the smallest values for k1 can be produced optically during the lithographic projection of dense structures, as has been shown in the publication xe2x80x9c170 nm gates fabricated by phase-shift mask and top anti-reflector processxe2x80x9d by T. Brunner et al. in Proc. SPIE, Vol. 1927 (1993), pp. 182-189, which is hereby incorporated into the disclosure content of the present application.
A particularly preferred type of design of the present invention consists in the double exposure with chromium-free phase masks of the above-described type, use being made, in relation to a first chromium-free phase mask, of a second chromium-free phase mask. The latter is arranged in relation to the first phase mask such that its arrangement of strip-like first and second areas assume an angle, preferably 90xc2x0, with respect to the arrangement of strip-like first and second areas of the first phase mask. The exposure can thus be carried out in a single exposure step with the photomasks placed one above the other. Alternatively, a single photomask can also be used, being used in two successive exposure steps and rotated through the aforementioned angle after the first exposure step.
Depending on the use of a positive or a negative resist system, either the exposed or the unexposed areas can be removed in a development step after the exposure steps, so that either a regular cluster arrangement or an equally regular hole arrangement can be obtained in a resist layer. The use of negative resists leads to contact holes, the use of positive resists leads to clusters or dots.
The method according to the invention can then be developed in such a way that, with the aid of the materialxe2x80x94in particular photoresistxe2x80x94structures produced, a corresponding arrangement of memory components such as magnetic memory components is produced. Here, first of all a matrix-like arrangement of microscopic photoresist structures is produced according to claim 1 and then, with the aid of the arrangement of photoresist structures, the arrangement of magnetic memory elements is produced. For example, first of all a matrix-like arrangement of depressions can be produced in a resist layer in accordance with the method of the invention, which is then filled with a suitable magnetic material in a deposition process. Following the removal of the resist material surrounding the filled depressions, there remains a regular arrangement of magnetic cluster structures, whose extent is determined by the extent of the depressions produced in the method according to the invention and which can be used as magnetic memory elements.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a photomask and its use for lithographic structuring of a photoresist layer and for producing magnetic memory elements, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.